What is a pulsioximeter?

It is non-invasive measurement of oxygen-carrying hemoglobin inside the blood vessels. It is done with a device called a pulse oximeter.

Knowing the state of oxygenation is an enormously effective method in our daily work. All patients in the ICU, have placed a translator of pulse oximetry. This device measures the arterial oxygen saturation by a curious system that measures the length between two points.

Although pulse oximetry is used to determine the degree of oxygenation of the blood, can not determine the metabolism of oxygen or the amount of oxygen you are using a patient

Usually for the measurement of oxygenation use a sample of arterial blood. This is an invasive technique. It requires placing a catheter inside an artery (usually radial), not without risks (hematoma, infection by germs that invade the catheter, etc).

The concept of pulse oximetry is not new. Carl Matthes in 1935 built the first device to continuously measure oxygen saturation in the blood in vivo by transillumination of the tissues.

How does a pulse oximeter?

The device emits light at two wavelengths of 660 nm (red) and 940 nm (infrared) that are characteristic respectively of oxyhemoglobin and reduced hemoglobin. Most light is absorbed by the connective tissue, skin, bone and blood vein by a constant amount, producing a small increase of this absorption in arterial blood with each beat, which means it requires the presence of arterial pulse for the device to recognize a signal. By comparing the light absorbed during the pulse wave absorption over baseline, calculate the percentage of oxyhemoglobin. Only measures the net absorption during a pulse wave, which minimizes the influence of tissue, veins and capillaries in the result.

The pulse oximeter measures the oxygen saturation in tissues, has a transducer with two parts, a light emitter and a photodetector, usually in the form of clamp and is usually placed on the finger, then wait to receive information on the screen: oxygen saturation, heart rate and pulse curve.

The correlation between oxygen saturation and PaO2 is determined by the dissociation curve of oxyhemoglobin.

LIMITATIONS

The pulse oximetry only measures oxygenation not ventilation, and not a substitute for blood gases by a laboratory, because it gives no indication on the base deficit, carbon dioxide levels, blood pH or bicarbonate concentration of HCO3 to does make the blood gases. The metabolism of oxygen can be measured by the determination of CO2 in expired air (capnography). The figures do not give any information saturation on the total content of oxygen in the blood (just how much amount of hemoglobin is saturated). Most of the blood oxygen is transported by hemoglobin, but in severe anemia, the blood carries less oxygen in total, although hemoglobin is saturated to 100%.

Other limitations of pulse oximetry

Changes in hemoglobin (MetHb or COHb).

Improper sensor placement.

Dyes and pigments in the reading area (nail polish).

Calloused skin

External light sources.

Peripheral hypoperfusion, such as when the patient is in shock!

Anemia.

Increased venous pulse.

It does not detect hyperoxia (O2 poisoning), as the pulse oximeter saturation to 100% limit. The air we breathe is a mixture of various gases including nitrogen (79%) and oxygen (20.97%) are almost 100% of it. For much more of O2 we apply, no more the Hb is saturated.
It does not detect hypoventilation. For this it is best to use the capnograph.

How do I put a finger?

MORE CONSIDERATIONS

CLINICAL INTERPRETATION

The devices available today are very reliable for values ​​between 80 and 100%, but its reliability falls below these figures.

Relationship between O2 saturation and PaO2

O2 SATURATION

PaO2

100 %

677

98,4 %

100

95 %

80

90 %

59

80 %

48

73 %

40

60 %

30

50 %

26

40 %

23

35 %

21

0 %

18

There is a critical value: PaO2 59 mm Hg corresponding to a saturation of 90%, below which small decreases in PaO2 cause significant desaturation. By contrast, over 95%, large increases in PaO2 not involve significant increases in oxygen saturation. To all this must take into account factors such as age, body temperature, in which oxygenation needs are higher than the general population.

In other clinical conditions, one must take into account that patients with COPD (chronic obstructive pulmonary disease) and severe chronic bronchitis, patients can raise SatO2 more than 90% can lead to a worsening of pulmonary ventilation by reducing the single stimulus with these patients is hypoxia. But this talk about what we do on the capnography!

INTERPRETATION OF Pulse oximetry oxygenation NOT DISCARD KNOW THE REST OF ARTERIAL GAS, SO THAT IS A RELIABLE METHOD OF MONITORING THE PATIENT, BUT NOT CONTROL THE SITUATION IN THE WHOLE OF THE oxygenation and ventilation of patients (THIS IS is done with the arterial blood gas)

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